Transparent, conductive tin oxide coatings on substrates such as glass, are widely employed for windshields, display devices, faceplates of cathode ray tubes, liquid crystal cells and the like. For the fabrication of faceplates of cathode ray tubes or vidicon faceplates, for example, electrical conductivity, measured as resistivity, should be on the order of less than 50,000 ohms per square and transparency, measured as percent light transmission in the visible range, should be over 80%. The most widely employed commercial process for fabricating tin oxide coatings is based on the pyrolysis of tin halide compounds. A compound such as stannic chloride is dissolved in a solvent and the solution sprayed onto the surface of a preheated substrate where the stannic chloride is pyrolized to form tin oxide. This method is inexpensive but is difficult to control to fabricate uniform, haze-free coatings, probably due to the action of hydrochloric acid, a by-product of the reaction, on the surface of the substrate.
The pyrolysis of organotin compounds has been tried. U.S. Pat. No. 2,567,331 discloses a method of applying a conductive, transparent coating of tin oxide to a glass substrate by spraying liquified dibutyl tin diacetate onto the glass which was preheated to its melting point. The resultant coating, while electrically conducting, had a less than satisfactory transmission of 58% and a high reflectivity of 20%. Some improvement was obtained by spraying the tin compound dissolved in an alcohol solvent, when transmission was increased and reflectivity was decreased. Very high spray temperatures are required for this process, however, on the order of 600-700.degree.C., which severely limit the substrates that can be employed and largely preclude the use of inexpensive, ordinary glass substrates. Another disadvantage of this process is that the tin oxide coating had a brownish appearance.
U.S. Pat. No. 3,107,177 discloses an improvement in the above process whereby fluorine is introduced into the tin oxide coating. The resultant tin oxide coatings have improved conductivity, but again, high temperatures are required which result in distortion or warping of low melting substrates such as glass.
Transparent, conductive coatings of tin oxide and indium oxide mixtures are employed in electro-optic devices such as liquid crystal display panels. The most widely used techniques for depositing this type of coating are radio frequency sputtering and electron beam evaporation. These techniques produce excellent coatings but require costly equipment, high electrical consumption and have a relatively low production rate.
Thus, a process for preparing transparent, haze-free, conductive coatings of tin oxide or tin oxide and indium oxide mixtures which is rapid and inexpensive and can be carried out at low temperatures would be highly desirable.